Cyclotron beam extraction

ABSTRACT

A method and apparatus for extraction of high beam currents from an isochronous cyclotron which segregates that portion of the orbiting particles to be extracted at the extraction radius in the absence of any electric deflection field and then electrostatically deflects the segregated particles from orbit with an electrostatic field the location of which increases in radius with respect to the center of the cyclotron. Segregation is made by a long sloped and cooled preseptum preceding an electrostatic deflection channel.

United States Patent [191 Hendry et al.

[ 51 Apr. 3, 1973 [54] CYCLOTRON BEAM EXTRACTION [75] Inventors: George 0. Hendry, Napa, Calif.; Dale K. Wells, Martinez, both of Calif.

[7 3] Assignee: The Cyclotron Corporation,

Berkeley, Calif.

[22] Filed: Feb. 25, 1971 [21] Appl. No.: 118,751

[52] US. Cl ..313/62, 313/63 [51] Int. Cl. ..ll05h 13/00 [58] Field of Search ..3l3/62, 63; 328/234- [56] References Cited UNITED STATES PATENTS 3,582,700 6/1971 Hendry ..313/62 Primary Examiner-Roy Lake Assistant ExaminerDarwin R. l-lostetter Attorney--Eckhoff, l-loppe, Slick, Mitchell & Ander' son [57] ABSTRACT A method and apparatus for extraction of high beam currents from an isochronous cyclotron which segregates that portion of the orbiting particles to be extracted at the extraction radius in the absence of any electric deflection field and then electrostatically deflects the segregated particles from orbit with an electrostatic field the location of which increases in radius with respect to the center of the cyclotron. Segregation is made by a long sloped and cooled preseptum preceding an electrostatic deflection channel.

6 Claims, 7 Drawing Figures PATENTEDAPR3 1975 3,725,709

sum 1 [1F 2 INVENTORS 52 52 GEORGE o. HENDRY ALE K ATTORNEYS PATENTEDAPR3 I973 SHEET 0F 2 INVENTORS GEORGE o. HENDRY AL K. WELLS BY l,- I

ATTORNEYS CYCLOTRON BEAM EXTRACTION This invention relates generally to beam extraction from isochronous cyclotrons and more particularly to a method and means for extracting a high quality beam with increased power.

One object of this invention is to increase the amount of high quality beam current obtainable from an isochronous cyclotron.

Another object of this invention is to provide a method and means for physically segregating that portion of orbiting particles to be extracted within the cyclotron at the extraction radius in the absence of any electric deflection field preliminary to their exposure to an electrostatic deflection field.

Still another object of this invention is to provide an improved method and means for reducing the power density on and for dissipating theheat generated in electrostatic deflection systems.

One other object of this invention is to enhance uniform beam deflection within the electrostatic deflection channel by preliminary particle segregation over a long sloped preseptum in the absence of any electric deflection field.

Other objects and advantages of this invention will become apparent from a consideration of the following description in connection with the drawings wherein FIG. 1 is a partially schematic cross-sectional view of a typical isochronous cyclotron at its median plane;

FIG. 2 is a plan view of the preseptum and electrostatic deflection assembly of this invention;

FIG. 3 is an end elevational view of the preseptum and electro-static deflection assembly of FIG. 2;

FIG. 4 is a plan view of the preseptum unit only;

FIG. 5 is an end view partially in section of the preseptum unit of FIG. 4;

FIG. 6 is an elevational view of the preseptum unit of FIG. 4 from the cyclotron central region; and

FIG. 7 is a partial sectional view of the preseptumunit taken along lines 7 7 of FIG. 2 to illustrate its locating means.

The external beam from isochronous cyclotrons generally is limited by high power density on the electrostatic extraction system. Any attempt to increase the external beam current increases the power density on the septum until it is destroyed by heat generated from accelerated particles impinging upon it. The septum power densities in isochronous cyclotrons are particularly high in the first place because of their characteristic small internal beam height.

This invention relates to the electrostatic extraction method and apparatus for isochronous cyclotrons disclosed in co-pending application, Ser. No. 775,027 entitled cyclotron beam extraction system filed on Nov. 12, 1968 by George 0. Hendry, now US. Pat. No. 3,582,700. The external beam current of isochronous cyclotrons therein described can be markedly increased by preliminary segregation of the orbiting particles to be extracted with a preseptum in the absence of any electric deflection field followed by electrostatic deflection of those segregated particles as is therein described. In this manner the heat generated by impingement of orbiting particles is taken by the preseptum where it is effectively distributed and dissipated. More uniform deflection then occurs in the electrostatic deflection channel since it need not be designed with an impingement notch.

FIG. 1 illustrates the orientation of the electrostatic deflection means and preseptum of this invention with respect to the other components of a typical small isochronous cyclotron at its median plane. A main d-c electromagnet defines a magnetic guide field for the particles orbiting within the evacuated region of the cyclotron. The main d-c magnet includes a lower yoke slab 2, a pair of interconnecting iron legs 3 and two cylindrical iron pole bases 4, the lower one of which is shown in FIG. 1. In the described embodiment the lower pole tip is warp plate 6b which with sidewalls and with a similar upper warp plate form a vacuum tank within which charged particles are accelerated in the machine.

Three shaped hill pieces 9 mount on each of the warp plates in corresponding locations to produce the azimuthally varying field necessary for isochronous operation. A pair of hollow 120 dees within the vacuum tank provides a radio frequency accelerating field and an ion source supplies ions for acceleration in the central region of the cyclotron between the two dees all as is more fully described in copending application Ser. No. 775,027.

In this embodiment the extraction system comprises electrostatic deflection means 30 like that shown in copending application Ser. No. 775,027 and a magnetic channel 31 which receives and radially focuses a beam of ions deflected by the electrostatic deflection means. Also a preseptum unit 50 mounts adjacent to the electrostatic deflection means 30 at the extraction radius and precedes it with respect to the path of orbiting particles.

FIGS. 2 through 7 show the preseptum unit 50 in detail and its orientation with respect to the electrostatic deflection means 30. The latter comprises a thin curved tungsten septum 35 maintained at ground potential and a curved deflector electrode 36 which is held at a high negative potential (for positively charged accelerated particles). The septum -35 and deflector electrode 36 define between them a shaped electrostatic channel 37 with a high electric field gradient which is located so it increases in radius with respect to the center of the cyclotron. When traversed by the segregated orbiting particles the field forces the ions to move to a larger radius where they no longer are held to a circular path by the main magnet. With the preseptum unit in place the septum need not have a notch to distribute particle impingement.

The septum and deflector electrode are carefully shaped and located so that the extracted beam remains centered in the channel as it moves to greater radius. Both mount upon a water cooled nonmagnetic base plate 38 pivotally mounted on one of the hill pieces 9 as shown in FIG. 1. The septum 35 clamps to the base plate 38 and an upper cooling plate 39. A pair of alumina insulators cantilevered from brackets 47 support the hollow copper deflector electrode 36 from base plate 38. Electrode 36 is cooled by water coolant supplied to its interior through hollow electrical conductor 48a and returned through conductor 48b. The same conductors 48a, 48b provide the high electrical poten- While output beam currents in the order of 50 microamperes of 22 MeV protons can be obtained with the described electrostatic deflection means alone, marked improvement in external beam current is obtained by using the preseptum for preliminarily segregating particles to be extracted prior to their entry into electrostatic channel 37. The preseptum unit 50 shown in detail on FIGS. 4, and 6 includes a thin curved tungsten preseptum 51 brazed or soldered to preseptum base plate 52 which in turn mounts upon base plate 38 of the electrostatic deflection means 30. The leading edge designated 53 of the preseptum 51 as shown in FIG. 6 slopes upwardly with respect to the cyclotron median plane in the direction of the orbiting particles. Preseptum 51 is maintained at ground potential and intercepts and segregates that portion of the orbiting particles at the extraction radius which are to be deflected from orbit by the electrostatic channel 37. The orbiting particles normally are distributed about the median plane and the slope of the leading edge 53 distributes beam impingement along a considerable length of preseptum to reduce the impingement power density and to facilitate heat removal from the preseptum. Accordingly, the entry portion of the leading edge is on one side and its trailing portion terminates on the other side of the median plane of the machine.

Cooling conduit 54 brazed to preseptum 51 carries cooling water communicated to it through cooling water conduits 55 connected to channels in preseptum base 52 that are in series with the cooling water circulating system for the electrostatic deflector. Coolant from inlet line 56 passes through base plate 38 and an O ring seal to the channels in preseptum base 52 and through preseptum cooling conduit 54. It then returns to the preseptum base, emerges through the O-ring seal 57 shown in section in FIG. 3 into conduit 58 which carries it to conduit 59 and the deflector means system for final return at outlet line 60.

Water cooled copper beam dump 61 mounted on preseptum base 52 and interconnected with the cooling water system in the preseptum base shields the deflector electrode 36 of the electrostatic deflection means from beam impingement.

In the embodiment shown the preseptum unit 50 is secured to the electrostatic deflection means base plate 38 by a miniature toggle clamp 62. Its orientation with respect to the electrostatic deflection means is maintained by locating pins 63.

With a preseptum as described a normal beam current of 50 microamperes of 22 MeV protons is increased to the order of 100 to 150 microamperes of 22 MeV protons.

It should be understood that the specific embodi- 5 ments described herein are for illustrative purposes only and that it will be apparent to those skilled in the art that various modifications may be practiced and equivalents substituted for those specific elements described which are within the scope of the invention defined in the appended claims.

We claim 1. A method for extracting a beam of charged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region comprising the steps of continuously segregating that portion of the orbiting particles to be extracted at an extraction radius near the fringe of said magnetic field in the absence of any electrostatic deflection field; and then deflecting he segregated particles from orbit at said extraction radius by exposure to an electrostatic deflection field the location of which increases in radius with respect to the center of the cyclotron. 2. Improved apparatus for extracting a beam of charged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region comprising means segregating that portion of said orbiting particles to be extracted at an extraction radius near the fringe of said magnetic field in the absence of any electrostatic deflection field; and

electrostatic deflection means receiving said segregated particles and defining at the extraction radius an electrostatic deflection field, the location of which increases in radius with respect to the center of the cyclotron, to deflect the segregated particles from orbit.

3. The apparatus of claim 2 wherein the means segregating the particles is a sloped and cooled preseptum adjacent to and preceding the electrostatic deflec- 40 tion means in the path of said particles.

4. The apparatus of claim 3 wherein the preseptum slopes from one side of the median plane to the other.

5. The apparatus of claim 3 wherein the preseptum is at ground potential.

6. The apparatus of claim 3 further comprising a beam dump shielding the deflector electrode of the electrostatic deflection means from beam impingement. 

1. A method for extracting a beam of charged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region comprising the steps of continuously segregating that portion of the orbiting particles to be extracted at an extraction radius near the fringe of said magnetic field in the absence of any electrostatic deflection field; and then deflecting the segregated particles from orbit at said extraction radius by exposure to an electrostatic deflection field the location of which increases in radius with respect to the center of the cyclotron.
 2. Improved apparatus for extracting a beam of charged particles orbiting in a cyclotron within its guiding magnetic field and evacuated region comprising means segregating that portion of said orbiting particles to be extracted at an extraction radius near the fringe of said magnetic field in the absence of any electrostatic deflection field; and electrostatic deflection means receiving said segregated particles and defining at the extraction radius an electrostatic deflection field, the location of which increases in radius with respect to the center of the cyclotron, to deflect the segregated particles from orbit.
 3. The apparatus of claim 2 wherein the means segregating the particles is a sloped and cooled preseptum adjacent to and preceding the electrostatic deflection means in the path of said particles.
 4. The apparatus of claim 3 wherein the preseptum slopes from one side of the median plane to the other.
 5. The apparatus of claim 3 wherein the preseptum is at ground potential.
 6. The apparatus of claim 3 further comprising a beam dump shielding the deflector electrode of the electrostatic deflection means from beam impingement. 